1. Field of the Invention
The present invention relates to an extremely low temperature refrigerator, and, more particularly, to a gas swing type refrigerator capable of reducing the electric power consumption at the time of its operation.
2. Description of the Prior Art
As disclosed in Japanese Utility Model Laid-Open No. 61-203267, conventional refrigerators, for example, a helium refrigerator or a helium liquefier have been arranged to generate a desired lowest temperature and a refrigeration quantity by cooling operating fluid for use in the cooling operation from ordinary temperature by a circuit constituted by combining a coldness generator and a heat exchanger before it is expanded by a Joule-Thomson (JT) valve.
In a case where the expander is arranged to act basing upon a Gifford-Macmahon cycle, a Solvay cycle or a Stirling cycle or the like, the refrigerator is arranged in such a manner that the operating fluid, for example, helium gas, for the expander is insulated from helium gas in a JT circuit comprising a heat exchanger and a JT valve. In the JT circuit, hot and high pressure helium gas is cooled in the heat exchanger by the returned coldness and low pressure helium gas and the same is further cooled by the coldness of the expander before it is introduced into the JT valve. The heat exchanger for exchanging heat between the high pressure helium gas and the low pressure helium gas in the JT circuit usually comprises a counterflow type heat exchanger. The quantity of coldness to be generated and necessary for the expander considerably depends upon the efficiency of the above-described counterflow type heat exchanger. If the efficiency is unsatisfactory, the temperature of the high pressure helium gas at the outlet portion of the heat exchanger is excessively raised. Therefore, the heat quantity to be cooled by the expander which is disposed downstream from the heat exchanger is necessarily increased. As a result, a large quantity of coldness must be generated.
The temperature effectiveness (or the heat exchange effectiveness) of an ordinary counterflow type heat exchanger is about 0.95, which is an insufficient value.
Furthermore, since the hot and high pressure helium gas flows in a predetermined direction in the counterflow type heat exchanger, impurities such as oil, water and air and the like which have been mixed into the helium gas by the action of the compressor which serves as the source for supplying the helium gas and which is disposed in the ordinary temperature portion are undesirably accumulated in the heat exchanger.
According to the above-described conventional technology, since the counterflow type heat exchanger is employed, the obtainable temperature efficiency .eta. is insufficient. Therefore, the quantity of the coldness to be generated in the expander becomes necessary in proportional to (1-.eta.). As a result, power to operate the expander must be increased so as to obtain the necessary quantity of the coldness. Therefore, a problem arises in that the power necessary to operate the refrigerator becomes too large.
Furthermore, according to the conventional technology, another problem takes place in that the impurities contained in the gas solidify and adhere to the heat transmissive surface the temperature of which is relatively low in the heat exchanger, causing the heat effectiveness of the heat exchanger to be further deteriorated. Another problem arises in that the same stops the fluid passage so that the flow of the helium gas is obstructed. Therefore, the necessary power to operate the expander must be further enlarged. In addition, the fact that the fluid passage has been clogged causes an insufficient gas flow to take place in the JT circuit. As a result, a problem arises in that the desired lowest temperature to be reached and the quantity of refrigeration cannot be realized.